Generally speaking, it has been a lot of work to remove all the non-JPA Hibernate stuff from the code before I could even compile with EclipseLink. Typically, these have been cache annotations (which I will now need from EclipseLink since standards are poor in this area), and I had some instances of ScrollableResult that I had to remove.
Hibernate seemed to apply more defaults than EclipseLink does. I had to add @Temporal to my date fields, whereas Hibernate was happy with a plain date. For queries I had to touch almost all of them while adding aliases, whereas Hibernate was happy without (select from Class C where c.name vs. select from Class where name). All in all it took me about a day for a project with around 100 queries and 50 domain objects. Now I am investigating the multi-tenancy features of EclipseLink, and I think I will give some feedback when I get feedback on my bugzillas :)